The present invention relates to dynamic measurement of runout of a rotating member, and more particularly concerns precision measurement of wobble of a high speed member while it rotates.
High speed drilling machines employed in the production of holes in circuit boards and similar items often use very small bits, having a diameter as small as 4 mils, operating at very high speeds, up to 100,000 rpm or greater. Bit breakage is common with such machines, but causes of the breakage are not always known with certainty. Wobble of the bit as it rotates at very high speed is a significant factor in bit breakage. Spindle runout is commonly measured at low rotational speeds by an instrument having a sensitive arm that physically contacts a precision drill rod as it is slowly rotated to provide a readout on a gauge of the amount of runout. The drill rod or pin replaces the bit for measurement of spindle runout. Such instruments, do not provide useful measurement of runout at bit operating speed, at which speed runout may be considerably different than it is at low speed. A capacitative system has been employed for dynamic runout measurement, including an arrangement for measuring capacitance between one side of the rod and a capacitive sensor on the instrument. Such systems are complex, costly, and subject to inaccuracies because of susceptibility to extraneous vibration. Known magnetic systems often employ differential transformers or permanent magnets but fail to achieve adequate precision of measurement. Measurement with precision of a few millionths of an inch is desired for optimum dynamic runout measurement, but heretofore has not been available with reliability or relatively simple, easily operated apparatus.
Accordingly, it is an object of the present invention to provide for dynamic runout measurement by apparatus and methods that provide simple, precise, and repeatable measurements of high accuracy.